Petroleum coke is the residue resulting from the thermal decomposition or pyrolysis of high boiling hydrocarbons, e.g. residual hydrocarbons with initial boiling points of 480.degree. C. or higher. High boiling virgin petroleum residues are typical feedstocks for the production of anode grade coke, the process often being carried out as an integral part of the overall petroleum refinery operation. Petroleum coke is manufactured by methods well known in the art, a major source being the delayed coking process (Bacha, J. D.; Newman, J. W.; White, J. L., eds., Delayed-Coking Process Update, PETROLEUM-DERIVED CARBONS, 1986, at 155). Other conventional coking methods known in the art include fluid coking and flexicoking.
Petroleum coke suitable for anode manufacturing is calcined in a rotary kiln at temperatures between 1200.degree. C. and 1400.degree. C. which results in the removal of excess water and volatile matter and densifies the carbon matter. The calcined coke is usually quenched with water and then formed into anodes for the production of aluminum.
Aluminum is produced by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte. The electrolytic cell, known as the Hall-Heroult cell, is typically a shallow vessel, with a carbon floor forming the cathode, the side walls comprising a rammed coal-pitch or coke-pitch mixture, and the anode consisting of a carbonaceous block suspended in the molten cryolite bath.
The anode is typically formed from a pitch-calcined petroleum coke blend, prebaked to form a monolithic block of amorphous carbon. The cathode is conventionally formed from a prebaked blend of pitch and calcined anthracite or coke, with cast-in-place iron over steel bar electrical conductors in grooves in the bottom of the cathode. A large electric current is passed through the molten bath between these two sets of electrodes and breaks down the dissolved alumina into aluminum and ionic oxygen. The molten aluminum collects at the bottom of the cell and is siphoned off after a sufficient amount accumulates. The oxygen reacts with the carbon at the anode to form carbon dioxide gas. The carbon anodes are replaced after the oxygen substantially consumes them.
In principle, when alumina is reduced to aluminum metal by the Hall-Heroult process, 0.33 pounds of carbon (coke) should be consumed for each pound of aluminum metal produced. In practice, however, more than 0.33 pounds of carbon are consumed per pound of aluminum produced. Although there are several different factors which contribute to these excess carbon losses, one of the most important factors is carbon airburn, i.e. the reaction of ambient oxygen at the exposed top surface of the anode: EQU O.sub.2 +C.fwdarw.CO.sub.2
Since the estimated capital loss to the aluminum industry due solely to excess carbon usage is quite significant, a modest reduction in the air reactivity of the anode can have a substantial impact in cost savings for the aluminum industry.
One of the major requirements of petroleum coke used in the production of carbon anodes is low metallic impurities. As increased usage of lower grade crude oils occurs, the availability of quality feedstocks for anode grade coke production has been diminishing. Increases in the metallic impurities content of petroleum coke produced from such crude oils can thus be expected because the impurities concentrate in the petroleum coke during coking operations.
High levels of metallic impurities adversely affect anode performance because the metals catalyze oxidation of the anode surface exposed to the atmosphere during high temperature cell operation. This results in airburning that adversely affects anode life. The oxidizing metal impurities found in petroleum coke often include, but are not limited to vanadium, sodium, nickel, calcium, and iron. The oxidation of petroleum coke by reaction with air at high temperature may be measured in the laboratory by procedures known in the art as tests for air reactivity (see Hume, S. M.; Fischer, W. K.; Perruchoud, R. C.; Welch, B. J., A Model for Petroleum Coke Reactivity, LIGHT METALS, 1993, at 525).
The use of magnesium-based materials to passivate metal impurities in petroleum coke has been described in U.S. Pat. No. 4,427,540. However, other useful materials that can passivate the metal impurities in petroleum coke are needed. Some aluminum producers attempt to inhibit carbon airburning by protecting the exposed anode surface by coating it with alumina or other compounds or burying it with alumina after positioning the anode in the cell. This method is not fully successful. Other methods involve surface treatment of calcined petroleum coke with a coating to reduce carbon airburning of the anode formed from the coke (U.S. Pat. No. 5,628,878). These methods, however, do not alter the intrinsic oxidation properties of the coke, and once the surface coating is lost, the exposed carbon is left without protection and the anode resumes airburning rates typical of its contaminant metal content. Thus, a need exists in the field of manufacturing anode grade coke to develop new processes for manufacturing the coke in a state where the metal contaminants are passivated.